PC2100 refers to Dual Data Rate (DDR) RAM that is designed to
run on 133Mhz system busses the name comes from the data transfer rate: On DDR systems the memory system is able to transfer data twice per clock cycle (compared to once per clock cycle on conventional memory ).
On current systems with a 64bit wide memory bus this gives the following maximum data transfer rates:
PC2100 : 64bits = 8 bytes and
8 Bytes * 133Mega (133,000,000) * 2 = 2128 MBytes/sec
which is referred to as PC2100
PC1600 refers to DDR memory designed to run on 100Mhz system
busses
PC1600 : 64bits = 8 bytes giving max data transfer rate of
8 Bytes * 100Mega (100,000,000) * 2 = 1600 MBytes/sec